(1) Field of the Invention
The present invention generally relates to an air bag system, and more particularly to an air bag system in which the inflating time of an air bag is determined based on an acceleration signal supplied by an acceleration sensor.
(2) Description of the Related Art
An air bag system inflates an air bag at a position between the driver and the steering wheel when the automobile collides with an object. The driver is protected from the shock of the collision by the absorption of the energy of motion of the driver by the air bag. The air bag system comprises an air bag driving mechanism and an air bag control system. The air bag driving mechanism is a mechanism which inflates an air bag. The air bag control system detects the automobile collision and judges whether or not inflation of an air bag is needed. If it is judged that inflation of the air bag is needed, the air bag control system controls the air bag driving mechanism to inflate the air bag at an appropriate time.
As a conventional air bag control system, there is disclosed an air bag system, for example, in the Japanese Laid-Open Patent Application No. 3-114944. The air bag system disclosed in the above application takes advantage of the difference of shock forces between a light collision and a pole collision and appropriately inflates the air bag by recognizing the mode of collision on the basis of the difference of shock forces.
Specifically, automobile speed information B is obtained by integrating an acceleration signal G supplied by an acceleration sensor, and a collision mode information value E is obtained by the difference between the maximum value G.sub.MAX and the minimum G.sub.MIN of the acceleration signal G in a predetermined period of time. Based on this information, a judging information value F is calculated by the following equation. EQU F=B+H*E (H: predetermined factor)
When the judging information value F obtained by the above equation is greater than, a predetermined threshold value K, it is judged that the collision is a pole collision (as will be explained in the following) and the air bag is inflated. On the other hand, if the judging information value F is less than the threshold value K, it is judged that the collision is a light collision and the air bag is not inflated.
Now, a description will be given of the collision modes with reference to FIG. 1A to FIG. 1E. FIG. 1A shows a front collision where an automobile 1 collides with an object 2a having a surface normal to the automobile direction of travel. FIG. 1B shows a slanting collision where the automobile 1 collides with an object 2b having a surface at an angle to the automobile direction of travel. FIG. 1C shows an offset collision where the automobile 1 collides with an object 2c offset from the automobile 1. FIG. 1D shows a pole collision where tile automobile 1 collides with a cylindrical object 2d such as a pole. FIG. 1e shows an under-ride collision where the automobile 1 collides with an object 2e positioned above the front bumper of the automobile 1.
As described above, there are various modes of collision. Accordingly, an acceleration signal output from the acceleration sensor varies depending on the mode of collision. The conventional air bag control system mentioned above has a problem in that although a determination of the pole collision and the light collision can be performed, other collision modes cannot be recognized.
If the collision mode cannot be recognized, there is a possibility that a driving signal is sent from the air bag control system to the air bag driving mechanism when it is not needed. Additionally, there is a possibility that the air bag is not inflated at an appropriate time because a driving signal is not sent at to the air bag driving mechanism at the time when it is needed.
Because of the structure of the conventional air bag control system, only two collision modes can be recognized by a single acceleration sensor. Accordingly, when recognizing all of the above mentioned collision modes, a plurality of sensors are needed. However, increasing the number of sensors results in an increase in manufacturing costs. Additionally, both hardware and software for a computer which processes signals supplied by each sensor becomes complex, and thus the cost of the system is further increased.